[xaprepare] Fix for newer homebrew on macOS (#5302)

Context: Homebrew/brew#9033
Context: Homebrew/brew@5adb76a

Homebrew as of Homebrew/brew@5adb76a5 no longer supports the use of
`brew ls` with the `--versions` and `-1` parameters:

	$ brew ls --versions -1 automake
	Error: Invalid usage: Options --versions and -1 are mutually exclusive.

This change in Homebrew breaks `xaprepare`, as `xaprepare` would
use `brew ls --versions -1` to determine if a program was installed:

	$ make prepare
	…
	Error: Invalid usage: Options --versions and -1 are mutually exclusive.
	 [MISSING]
	Checking automake                                   Usage: brew list, ls [options] [formula|cask]

The intention for our use of `xaprepare` was so that *if* a Homebrew
package had multiple versions installed, the use of
`brew ls --versions -1` would cause *only* the latest version to be
printed, not every version.

As `brew ls --versions -1` is no longer supported, update `xaprepare`
so that `brew ls --versions` output is parsed, preserving only the
first line of output.  This avoids the invalid usage error, while
preserving our desired semantics incase multiple versions are present.

Author: grendello

build-tools/xaprepare/xaprepare/Application/HomebrewProgram.MacOS.cs

@@ -6,6 +6,8 @@ namespace Xamarin.Android.Prepare
 {
 	class HomebrewProgram : Program
 	{
+		static readonly char[] lineSplit = new [] { '\n' };
+
 		string cachedVersionOutput;
 		bool brewNeedsSudo = false;
 		bool multipleVersionsPickle = false;
@@ -204,13 +206,21 @@ protected override async Task AfterDetect (bool installed)
 
 		string GetPackageVersion ()
 		{
-			return Utilities.GetStringFromStdout (
+			string output = Utilities.GetStringFromStdout (
 				Context.Instance.Tools.BrewPath,
 				false, // throwOnErrors
 				true,  // trimTrailingWhitespace
 				true,  // quietErrors
-				"ls", "--versions", "-1", Name
+				"ls", "--versions", Name
 			);
+
+			if (String.IsNullOrEmpty (output))
+				return output;
+
+			string[] lines = output.Split (lineSplit, StringSplitOptions.RemoveEmptyEntries);
+			if (lines.Length == 0)
+				return String.Empty;
+			return lines [0];
 		}
 	}
 }